Colin William Gray

Soil biology encompasses a vast diversity of organisms contributing to soil processes and functions that underpin the delivery of soil ecosystem services. We review the response of soil biology to long‐term phosphorus (P) and nitrogen (N) fertiliser application, focusing on trials conducted at Winchmore and Ballantrae Research Stations under sheep grazing. Application of P and N fertiliser increases soil fertility and pasture production and can have impacts on soil biology. Higher earthworm abundance was associated with increasing P, but not N fertiliser application. Microbial biomass or respiration tended to increase with P application but decrease with N application. Both P and N fertiliser decreased fungal biomass. The few microbial functional groups studied also appear responsive to fertilisation. A decline in soil pH may have driven some of these changes and could have been mitigated with lime. Other factors, such as plant composition, may also be important to the soil biology but have received limited attention in these studies. There are currently few indicators that can be used on‐farm to assess soil biology. We discuss these as well as measures that can be used in a research context to inform the impacts of fertiliser use on soil biology and its functioning.

Cadmium (Cd) has accumulated in New Zealand agricultural systems, predominantly from the application of phosphorus (P) fertiliser to soils. Elevated Cd concentrations in soils are an important issue due to their potential adverse effects on food quality, soil health, land use flexibility, and the environment. This paper reports on our current knowledge of Cd in New Zealand agricultural systems, summarising published and unpublished research that has been undertaken over the last 30 years to better understand, improve, and manage the potential adverse effects of Cd in agricultural soils. We found the research has been wide ranging, investigating factors such as Cd uptake in plants and animals, the rate of Cd inputs, transformation and losses from soils, Cd bioavailability, the development of animal uptake and Cd mass‐balance models, as well as identifying and testing strategies and remediation options to manage Cd accumulation in agricultural systems. This research is then placed alongside the policy and regulatory context for managing Cd in agricultural systems in New Zealand, including national Cd management strategies. Finally, key knowledge gaps are presented, along with some potential Cd research directions for the future.

Contaminant loss from grazed pasture can negatively affect freshwater quality. There is, however, little data on the impact of different levels of grazing/treading on contaminant loss measured under field conditions. This study quantified phosphorus (P), sediment and Escherichia coli ( E. coli ) loss in surface runoff from plots grazed by cattle for 0, 16 or 28 h to create different pasture/treading damage. Results showed an increase in grazing/treading duration decreased soil macroporosity (30%) and saturated hydraulic conductivity (96%), and increased surface roughness (71%) and depression water storage capacity (388%). These changes in soil physical condition contributed to greater volumes of surface runoff in the 16 h (31%) and 28 h (55%) treatments. Contaminant concentrations and loads in runoff also tended to increase with grazing/treading, although the increases were not statistically significant. An exception was total P (TP) which increased from 0.323 to 1.222 kg ha ⁻¹ , principally due to increased dissolved P, probably released from plants due to grazing and from the deposition of dung. Management of animals to minimise the number, duration and frequency of grazing wet soils that are vulnerable to physical damage are likely key factors to reducing TP, and probably SS and E. coli transport in surface runoff.

Mass balance modelling often shows that leaching is the main loss pathway for cadmium (Cd) from agricultural soils. There is however a scarcity of Cd leaching data measured under field conditions to support these modelled losses. This study used passive‐wick fluxmeters installed at a depth of one metre at six commercial cropping sites to measure Cd in drainage over one year of monitoring. Cadmium concentrations were generally below the method detection limit (0.05 µg Cd L ⁻¹ ) for all sites, with only the occasional detection from two sites and no detection from three other sites. The exception was site A where average Cd concentrations ranged from 0.06 to 0.18 µg L ⁻¹ and the mean annual loss (± SE) was 0.33 ± 0.08 g Cd ha ⁻¹ . This was similar to the amount estimated using the drainage flux multiplied by a modelled estimate of soil solution Cd concentrations for the site, 0.41 g Cd ha ⁻¹ . Cadmium balance models typically assess the rate of Cd accumulation in the 0–7.5 or 0–30 cm soil depth. Further studies are still required to obtain Cd leaching data collected under field conditions at these soil depths to allow modelled Cd leaching to be validated against measured data.

Cadmium (Cd) leaching is often estimated in Cd balance models using the product of drainage water (precipitation excess) and soil solution Cd concentration. However, Cd concentrations are seldom available but rather predicted using empirical models. Despite the availability of empirical models that estimate soil solution Cd concentrations, they have limitations for use in New Zealand where total Cd in agricultural soils is low and organic matter (OM) high. This study derived a Freundlich‐type model from desorption data and a soil–liquid partitioning coefficient (K_D) model based on sorption data to predict soil solution Cd concentrations from commonly measured soil parameters that could be used to calculate Cd leaching fluxes. Independent soil solution Cd concentrations and Cd fluxes measured in drainage water from field trials were used to validate the predictive capacity of the models. It was found that soil pH and OM content were the most important factors controlling soil solution Cd, along with total Cd. Both models explained 83% of the variation in measured soil solution Cd concentrations in an independent dataset. Comparisons between Cd fluxes predicted using the Freundlich‐type model and measured fluxes were within 25% of each other at 6 of 19 field sites studied. However, physical and chemical nonequilibrium conditions in soils and uncertainty in measured values likely contributed to differences between predicted and measured Cd fluxes at other sites. To unravel the impact of nonequilibrium and soil physical conditions on Cd concentrations in drainage water, more data are required on Cd concentrations collected under field conditions. This will allow better validation of the approach used in Cd balance models to calculate Cd leaching from soils.

The application of amendments such as farm dairy effluent (FDE) to soils which contain dissolved organic carbon (DOC) has the potential to increase cadmium (Cd) leaching through the formation of soluble organo-Cd complexes. However, the extent of Cd leaching loss is currently unknown. A lysimeter study measured Cd leaching from coarse and fine-textured Pumice soils amended with either FDE or single superphosphate (SSP) fertilizer. Results showed that despite FDE having a DOC concentration of 3000 mg L⁻¹, concentrations in drainage were low (7.2 to 14.6 mg DOC L⁻¹), probably reduced by microbes and/or sorption onto iron and aluminium oxides in the soil. As a result, there was no significant difference in the amount of Cd lost from the soil amended with FDE (0.79 g ha⁻¹) or SSP (0.70 g ha⁻¹). In comparison, greater amounts of Cd were lost from the fine-textured (0.92 g ha⁻¹) than from the coarse-textured soil (0.57 g ha⁻¹), primarily due to significantly higher Cd concentrations in drainage. The study indicates that a one-off application of FDE at the maximum rate allowed by regulators is unlikely to affect Cd leaching losses, although this still should be confirmed for other soil types that receive FDE.

The extent of phosphorus (P) loss from soils under deep rooting crops such as lucerne is currently unknown. This study used large lysimeters (2.0 by 1.5 m) to quantify the amounts and forms of P in drainage under lucerne from two sites, a non‐irrigated dairy system and an irrigated dairy system that also received farm dairy effluent (FDE). Results showed despite greater P inputs to the irrigated compared to the non‐irrigated site, there was no difference in total P concentrations in drainage. There were also no differences in the forms of P lost between sites, which were dominated by particulate P (48%–52%). More P was lost from the irrigated (0.262 kg ha ⁻¹ ) than the non‐irrigated site (0.164 kg ha ⁻¹ ). The larger P loss was a result of irrigation producing more drainage (42%) than observed at the non‐irrigated site. The amount and forms of P lost from lucerne were similar or lower than losses reported for similar soils under grass/legume‐based pasture swards amended with P fertiliser and FDE. Because P leaching was regulated by drainage volume rather than P input, management of irrigation water to reduce drainage from the root zone will likely help minimise P leaching.

Reliable data on the rate of cadmium (Cd) accumulation in soils that receive phosphorus (P) fertiliser is vital to help manage the impact of Cd in agricultural systems in New Zealand. Studies of the long‐term P fertiliser trial at Winchmore, where soils have received 0, 17, or 34 kg P ha ⁻¹ yr ⁻¹ for > 60 years, have reported that soil Cd concentrations over the last 15 years have either plateaued or are still increasing. To address this discrepancy, new soil Cd analysis was conducted on a wider set of archived soil samples from the Winchmore trial, along with some re‐analysis of soil samples for Cd. When soil Cd data from previous studies were combined with the more recent set of Cd data, regression analysis indicated soil Cd concentrations in the 17, or 34 kg P ha ⁻¹ yr ⁻¹ fertiliser treatments are still increasing linearly over time. It is recommended soil samples from the long‐term Winchmore trial continue to be analysed for Cd to confirm long‐term trends, avoiding short‐term perturbations. Such long‐term trials are invaluable for providing robust information that will help in the ongoing management of Cd in New Zealand.

Cadmium (Cd) can accumulate in soil from the application of phosphorus fertilizer. However, there is little information on what happens to soil Cd concentrations when Cd inputs stop. This study used soil and pasture samples collected from a long‐term field trial to measure changes in Cd concentrations in soil for 22 yr after Cd inputs from fertilizer had stopped and assessed whether the application of nitrogen (N) (50 kg ha⁻¹ yr⁻¹) could increase plant uptake of Cd and reduce soil Cd concentrations. It was found that there was no significant change in total or labile soil Cd (1 M CaCl₂ extractable) concentrations after Cd inputs stopped. The application of N did not significantly (P < .05) increase dry matter yield or increase Cd solubility. As a result, N did not enhance plant uptake of Cd. A mass balance that included Cd loss via plant uptake and Cd leaching confirmed they were insufficient to result in a detectable decrease in soil Cd concentration over the 22‐yr interval of the trial. It appears that even an acid soil with low amounts of carbon (2.67%), iron/aluminum oxides, and clay can still strongly retain Cd, preventing Cd depletion from the soil, despite stopping Cd inputs and trying to enhance plant uptake of Cd from the application of N fertilizer.